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Moderate Exercise Boosts Immunity, But Marathons Could Make You Sick

We’ve all heard that exercise can help stave off illness by offering an immune system boost, but what kind of activity is best? According to one expert who surveyed the research, moderate exercise — things like taking a brisk walk or playing touch football with friends — can reduce our risk of getting colds and flu viruses. But in a case of “less is more,” the same is not true of prolonged, intensive training, like the kind undertaken by marathon runners and elite athletes. Unlike an average workout, a marathon can actually increase the likelihood that an athlete will get sick.

A totally sedentary person is likely to contract a yearly average of two to three upper respiratory tract infections (URTIs) — the medical term for viral infections of the ear, nose and throat, like colds, flu and sinus infections. But a moderately active person can expect to reduce that rate by almost a third, according to Mike Gleeson, a professor of exercise biochemistry at Loughborough University in Leicestershire, U.K. By contrast, an elite endurance athlete who completes intensive training can expect two to six times as many URTIs during a year.

But why? Moderate exercise can enhance the activity of Natural Killer cells — a type of white blood cell that targets and immobilizes virally-infected cells. What makes endurance exercise different is its ability to cause a stress response, in which stress hormones like cortisol are released to make the intensive effort more bearable. According to Gleeson, stress hormones inhibit the activity of NK cells.

“In periods following prolonged strenuous exercise, the likelihood of an individual becoming ill actually increases. In the weeks following a marathon, studies have reported a 2-6 fold increase in the risk of developing an upper respiratory infection,” said Gleeson during his presentation to the Association for Science Education (ASE) Conference, where he represented the Society for General Microbiology and the British Society for Immunology. “The heavy training loads of endurance athletes make them more susceptible to URTIs and this is an issue for them as infections can mean missing training sessions or under-performing in competitions.”

Researchers have long known about the connection between intensive exercise and immunosuppression, but it’s a good reminder this month as many of us begin or ramp up our exercise routines as part of our New Year’s resolutions. A moderate approach may be healthiest.